Load sensing hydraulic systems use load generated pressure to control pump displacement and some pressure compensating valve functions. The fluid used for such control functions is usually exhausted to the tank through control orifices or pressure regulating valves. Typically, the load pressure is connected to a load pressure signal network through a signal flow path of a directional control valve when the directional control valve is moved to an operating position to direct fluid from the pump to a hydraulic motor. Using load generated pressurized fluid for those control functions can result in load drift under some operating conditions. For example, many industrial or earthmoving vehicles have two or more moveable components controlled by hydraulic motors. Some of those components are arranged such that movement of one component can induce in the hydraulic motor connected to another component a load generated pressure greater than the pressure capacity of the pump. If the directional control valve associated with the motor having such load generated pressure therein is moved to an operating position, pressurized fluid from the motor could flow through the load signal flow path of the directional control valve and be lost across the load signal relief valve.
One arrangement for solving that problem is disclosed in an SAE Technical Paper No. 891941 dated Sept. 11-13, 1989 and entitled "The Synchro Control System For Mobile Applications" which shows a pair of pressure compensating valves integrally disposed in bores of a main valve spool of a directional control valve with each of the pressure compensating valves having a shuttle valve integrally positioned therein. The shuttle valve provides a dual function of either directing the load pressure signal of a particular work circuit directly to the associated pressure compensated valve if that work circuit has the highest load pressure or for directing a pressure signal from another work circuit to the pressure compensating valve if the load pressure of the associated work circuit is lower than the load pressure of another of the work circuits. However, that valve is very complex and would be expensive to manufacture since that system requires a pair of pressure compensating valves and a pair of shuttle valves associated with every one of the directional control valves. The valve arrangement disclosed in the above-noted SAE paper is also disclosed in U.S. Pat. No. 4,719,753.
The present invention is directed to overcoming one or more of the problems set forth above.